I should have done better

Not sure where exactly I went wrong, but thought I had adjusted my tax being withheld so that I would only get a $100 (+/- 50) for a refund. However, that is not the case. I overpaid a whopping $1100. That's a large amount of $$$ to just for all intensive purposes "stick under a mattress." At first I thought I could attribute the oversight to all the stuff that I donated this past year when my girlfriend and I combined into one house and had to get rid of a LOT of stuff. Obviously it would be hard for me to plan for that when setting up my tax papers, but that only accounts for about $300 of my refund. So what exactly was different from last year that caused an $800 difference? The only thing I did differently is make MORE money.....which in turn means I should owe MORE money. Now don't get me wrong. I'm definitely not complaining about an $1100 "windfall."  It's just I'd rather have that windfall throughout the year so that I can at least earn interest on it (more than likely, I would just accelerate my debt payment that much faster). Oh well, this year should be more stable and give me an actual idea of where my numbers need to be.